Modern electronic copiers, printers, facsimile machines, etc. are capable of producing complex and interesting page images. The pages may include text, graphics, and scanned or computer-generated images. The image of a page may be described as a collection of simple image components or primitives (characters, lines, bitmaps, colors, etc.). Complex pages are built from these primitives using a page description language such as PostScript. The electronic printer's software converts these to an equivalent bit stream by a process called rasterization to build an internal, electronic model of the page. The data is contained in an image buffer in the form of an array of color values called pixels. A count of the number of pixels to be printed gives an estimate of the area of each page to be printed. The approach utilized for multicolor electrostatographic printing is substantially identical to the process described above. However, rather than forming a single latent image on the photoconductive surface in order to reproduce an original document, as in the case of black and white printing, multiple latent images corresponding to color separations are sequentially recorded on the photoconductive surface. Each single color electrostatic latent image is developed with toner of a color complimentary thereto and the process is repeated for differently colored images with the respective toner of complimentary color. Thereafter, each single color toner image can be transferred to the copy sheet in superimposed registration with the prior toner image, creating a multi-layered toner image on the copy sheet. Finally, this multi-layered toner image is permanently affixed to the copy sheet in substantially conventional manner to form a finished copy.
With the increase in use and flexibility of printing machines, especially color printing machines which print with two or more different colored toners, it has become increasingly important to monitor the toner development process so that increased print quality, stability and control requirements can be met and maintained. For example, it is very important for each component color of a multi-color image to be stably formed at the correct toner density because any deviation from the correct toner density may be visible in the final composite image. Additionally, deviations from desired toner densities may also cause visible defects in mono-color images, particularly when such images are half-tone images. Therefore, many methods have been developed to monitor the toner development process to detect present or prevent future image quality problems.
For example, it is known to monitor the developed mass per unit area (DMA) for a toner development process by using densitometers such as infrared densitometers (IRDs) to measure the mass of a toner process control patch formed on an imaging member. IRDs measure total developed mass (i.e., on the imaging member), which is a function of developability and electrostatics.
Electrostatic voltages are measured using a sensor such as an ElectroStatic Voltmeter (ESV). Developability is the rate at which development (toner mass/area) takes place. The rate is usually a function of the toner concentration in the developer housing. Toner concentration (TC) is measured by directly measuring the percentage of toner in the developer housing (which, as is well known, contains toner and carrier particles).
As indicated above, the development process is typically monitored (and thereby controlled) by measuring the mass of a toner process control patch and by measuring toner concentration (TC) in the developer housing. However, the relationship between TC and developability is affected by other variables such as ambient temperature, humidity and the age of the toner. For example, a three-percent TC results in different developabilities depending on the variables listed above. Therefore, in order to ensure good developability, which is necessary to provide high quality images, toner age must be considered.
The following disclosure may be relevant: U.S. Pat. No. 6,047,142 which hereby is incorporated by reference. That patent discloses that in order to ensure good developability, which is necessary to provide high quality images, toner age must be considered. In that patent a method for estimating toner age is described in which pixel count is used to estimate the amount of toner used to form a xerographic image.
Additionally, Applicants have found that it may be important to also monitor the age of the other component of the developer, the carrier. When carriers which are used in conductive or semiconductive magnetic brush development systems become encased in toner resin fines they may become too insulative to function properly, leading to poor development of solid areas. Alternatively, coatings on the carrier which are present to provide proper tribocharging of the toner, can wear off with the result that the carrier no longer functions as intended. The severity of either mode of degradation is proportional to how long the carrier has been in use, i.e. the carrier age. Monitoring the carrier age will allow one to take appropriate service actions based on the carrier age. Such actions may include, but are not necessarily limited to, adding extra raw carrier, to flush old material, using a special, high carrier content replenisher, or simply installing a new developer.
In some print engines and copiers, some carrier may be mixed with the toner which is added to replace that used in making prints or copies. (In general, this material, whether just toner or a mixture of toner and other components such as carrier will be called replenisher.) In these cases measurement of the carrier age must be made in a manner analogous to that used to measure toner age.
There is provided a method for estimating both toner age and carrier age based on measuring the amount of replenisher actually dispensed. This method is robust against errors which can arise from using pixel count as the basis for estimating the toner age or carrier age. In image-on-image development systems, the developed mass per unit area will depend on whether the developed toner is deposited directly on the photoreceptor or is deposited on toner developed in previous steps. Thus the average developed toner mass per unit area (dma) will depend on the image content, and thus be prone to error. These errors are compounded further by non-linear half toning effects. As an example, a 10% halftone will require developing 1/10th the available pixels in a given area, but the amount of toner developed under the same conditions used to develop the solid will, in general not be 10% of the amount required to develop all the pixels in that area. This departure from proportionality to the fraction of pixels developed will change with the proportion of pixels and may change with selected print conditions, such as darkness or lightness control settings. Additionally, the pixel count cannot account for non-printing toner usage such as emissions or while adding toner without developing (a tone-up process). A properly calibrated system for dispensing toner or replenisher material into a developer housing to replace the material removed provides an alternate and improved method for measuring toner age. In addition, it does not require the additional electronic circuitry associated with counting pixels. Because a pixel count is not used, the invention is also applicable to copiers and similar devices which do not have digital images.
There is provided a method for measuring and controlling developer age in a developer housing having developer including carrier and toner comprising: providing a maximum developer age in a memory; sensing toner concentration in the developer housing and storing toner concentration in the memory; calculating the amount of dispensed replenisher from the dispense rate and the toner in the replenisher; determining toner age, carrier age or the age of both components in the developer housing based upon the toner concentration, and the amount of replenisher dispensed since a previous age calculation; and interrupting a print job when the toner age is greater than a maximum toner age, when the carrier age is greater than a maximum carrier age, or when the component ages in combination is greater than some maximum value.
There is also provided an apparatus for measuring and controlling toner age in a developer housing comprising: a memory storing a maximum toner age, a maximum carrier age or both; a dispenser for dispensing replenisher to a developer housing at a dispense rate; determining toner age, carrier age or both in the developer housing based upon the toner concentration, and the amount of replenisher dispensed since a previous age calculation; and a toner concentration sensor sensing a toner concentration in the developer housing; a mass sensor sensing a developed mass per unit area; and a control unit receiving the dispense rate, the toner concentration and the developed mass per unit area, determining the toner age, the carrier age, or both ages in the developer housing based upon the dispense rate, and the toner concentration, and initiating a purging of the toner in the developer housing when the toner age is greater than the maximum toner age.